Wet process of classifying comminuted material



ll atented Mar, 313, 3%23.

UNHTEE TATEg aaasia aarssr @FWQZE,

WALTER O. BORCHERDT, OF AUSTINVILLE, VIRGINIA, ASSIGNOR TO THE NEW JERSEY ZINC COMPANY, OF NEW YORK, N. Y., A CORPORATION OF NEW JERSEY.

WET PROCESS OF CLASSIFYING COMMINUTED MATERIAL.

No Drawing.

To all whom it may concern-.-

Be it. known that I, WALTER O. BORCHERDT, residing at Austinville, in the county of VVythe, State of Virginia, have invented oer 5 tain new and useful Improvements in Wet Processes of Classifying Comminuted Material; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to improvements in wet processes of classifying comminuted material containing colloidal matter, such, for

example, as the hydraulic classification or wet screening of ore pulps or ore slimes. These wetclassifying processes, as generally used, are intended to segregate the solid constituents of a pulp containing the comminuted material into groups or classes in which the maximum and minimum diameters of the particles in any one class vary within smaller limits than in the original mixed pulp. lhe present invention contemplates certain improvements in wet processes of clasifying comminuted material in a pulp-containing colloidal matter. 1

Such processes of hydraulic classification and wet screening are used in the preparation of ore pulps for separation or concentration of the valuable minerals by jigging.

tabling, magnetic separation, flotation and the like, and also in connection with crushing or grinding of ore pulps to separate those particles which are sufficiently comminuted from those which require further treatment. While there is an almost infinite variety in the methods of applying both hydraulic classification and screening, and many appliances have been developed in order to facilitate the operations, all these methods and appliances depend upon the utilization of one or the other or a combination of two principles.

In hydraulic classification the solid particles of the ore p,ulp are subjected to the action of a risingicurrent of water having a velocity such that; whilst certain of the particles descend despite the influence of the rising current and are drawn off at one point, other particles, of smaller size, or lower specific gravity, are sustained by the rising current and are discharged at another Application filed January 5, 1920. Serial No. 349,572.

point. As is well known the segregation thus produced is a resultant of both the size and the specific gravity of the particles, so that the material which fallsthrough the rising current consists of relatively large particles of light minerals and of relatively small particles of heavier minerals.

Depending upon the conditions under' particles, while in the other type known as hindered settling classification, the operation is. so conducted that the particles crowd one another and their final disposition is determined by the velocity of flow of the interstitial water. Frequently these two kinds of classification. are employed in the "same piece of apparatus, the bulk of the material being treated under free settling conditions, while the material close to the point of division for which the apparatus is set is subjected to hindered settling conditions. Under hindered settling conditions there is. in general, greater differentiation of size between the large particles of light mineral and the smaller particles of heavier mineral which the apparatus segregates into a specific class.

In wet screening the segregation of particles into groups or classes is practically based-only upon their average diameter, although in some cases the specific gravity of the particles has a minor influence. In wet screening a perforated plate or fabric such as screen cloth composed of wires or bars provides a surface containing a large number of openings or orifices of determinate size, and the ore pulp is brought into contact with said perforate surface in such a. way that particles of suiliciently small diameter pass through the perforations and form one group or class, while those which fail to pass through are transported to another point.

In the preparation of ore pulps, use is frequently made of the methods of hydraulic classification and of screening, in combinatlon, sometimes because one or' the other process yieldsa more desirable product for the subsequent treatment to which the pulp is to be subjected, or because of the practical limitations which enter into employment of the processes. In the coarse crushing of ores to diameters approximating e or larger, screening is generally employed as the method of segregation, while in the fine grinding of ores to sizes less than approximately 30 mesh, hydraulic classification is more commonly employed, leaving a middle ground within the limits of about 2 as'the major size and 30 mesh as the minor size, within which the choice as between hydraulic classification and wet screening is determined by other considerations of greater weight than the choice of method of segregation.

In both hydraulic classification and wet screening the. value of the results secured depends upon the sharpness or degree of diflerentiation show-n by the separation of particles into classes and it is always particularly undesirable to have very fine material present in a class where it is intended that the bulk f the material shall be relatively coarse. Aside from the effects of the factors of mechanical imperfection and overloading, there is a very decided tendency on the part of fine material to stick to or remain associated with coarser particles, and my investigations lead me-to believe that to a considerable extent this undesirable association persists because of the presence in the ore pulp of colloidal or quasi-colloidal ingredients especially when these are ina flocculated or coagulated, condition. Floccules or coagules of colloidal matter may behave like solid particles of equivalent size, and these may pass into the spigot product of a classifier or the oversize of a screen, despite the fact that because they are only an aggregation of very small particles, they do not properly belong there; although the apparatus in so disposing of them has acted in accordance with the physical laws which govern it. Frequently, also, such coagules or floccules of colloidal material entangle or enmesh coarser particles of the pulp, further confusing the segregation which it is desired to bring about, and in addition colloidal material frequently adsorbs or otherwise attaches itself to granular material in such a way as to accompany it when a separation is desired. Such colloidal materials often have the power of holding to themselves and to the materials with which they are associated relatively large quantities of water, and when in a flocculated condition display a marked tendency toward clogging or obstructing the apertures in fine screens.

The colloidal constituents of ore pu lps may be either organic or inorganic and while it seems probable that the inorganic colloids ily charged with both organic and inorganic colloidal materialsrun into the stream from which the milling water is customarily taken. So also, such colloidal materials may occur in the milling water as the result of freshets or of ore dressing or other operations further up stream. The colloidal matters may be present as pure colloids, organic'or in-. organic, or in a state bordering on a socalled coarse suspension. Throughout this specification and the appended claims I intend to embrace in the expressions colloids,

colloidal matter and colloidal constituents, both true colloids and those quasi or borderland substances which may not be purely col-loidal but partake of the qualities of r resemble colloids, and without regard to the source from which such colloidal matters are derived. Thus, for the purposes of the present invention, the colloidal constituents may be considered as ranging from just above the molecular size upward through the class of so-called suspensoids, which,

while possibly not true colloids, still, be-' cause of their relatively large surface, exhibit some, if not all,"of the properties of colloids, and are capable f existing in both the sol and gel state, or of being dispersed or coagulated or flocculated and deflocculated. Bycolloidal matter, therefore, I mean material which is not necessarily a true colloid, but exhibits colloidal character; istics to a suflicient degree to make it amenable to the manipulation hereinafter d'e-- scribed.

The colloidal constituents of an .ore pulp rarely, if ever, naturally occur in a completely deflocculated and dispersed condition, and I have discovered that the interference with effective hydraulic classification or wet screening, which such colloidal ingredients of an ore pulp cause, is approximately proportional to their degree of coagulation orfiocculation. and that this interference can be reduced to any desired extent by a preliminary treatment whereby the colloidal constituents of the ore pulp are deflocculated or dispersed and removed from the pulp to such a degree that they no longer interfere in the subsequent process of hydraulic classification or wet screening,

The present invention, in its broad aspect, contemplates, as an improvement in the hy draulic classification or wet screening of ore pulps, and as a preliminary step to the classification or screening proper, the removal,

- in whole or in part, of colloidal constituents substantially alone from the ore pulp, and involves the step of dispersing or defiocculatin colloidal matter which would otherwise e present in the ore pulp in a flocculated condition. By removal of colloidal constituents substantially alone, I mean that the removal of the colloidal constituents is accompanied by no substantial removal from the ore pulp of the non-colloidal constituents therein, although there may be, and usually will be, removed from the pulp along with the colloidal constituents a por tion of the water forming the pulp. After the removal of the colloidal constituents fromv the ore pulp in accordance with the principle of the present invention, the remaining ore pulp'is subjected to a wet process of classification for eflecting the desired segregation of comminuted material therein.

The removal of colloidal constituents from the ore pulp may be efiected in various ways. Thus, for example, these constituents may be removed by sedimentation and decantation, and they may be removed by a flotation treatment conducted under such conditions as to inhibit the flotation of mineral particles, while permitting the removal of the colloidal constituents in the form of a colloid-bearing froth. After the removal of the colloidal constituents, in accordance with the principles of this invention, the ore pulp is subjected to hydraulic classification or screening in accordance with any of the usual practices.

The removal of the colloidal constituents from the mineral ore pulp by sedimentationand decantation depends for its effectiveness upon the relative difference in the rate of settling or sedimentation of the ore particles or mineral particles and the colloidal constituents. For this reason it is desirable to cause the colloidal constituents to settle at a much slower rate than the mineral and gangue particles in the ore pulp, whereby, after a predetermined settling of the mineral and gangue particles, any desired portion of the supernatant liquor containing the slower-settling colloidal constituents can be decanted in any suitable way to effect the removal from the ore pulp of the desired quantity of the colloidal constituents. In this connection, the present invention contemplates the dispersion or deflocculation of the colloidal constituents so as to cause them to settle at a very much slower rate than would bethe case if they were coagulated or flocculated, so that they will remain in more or less permanent suspension in quiet pulp,

defiocculating or dispersing agent in order.

to convert the colloids into a defiocculated or dispersed state and the dispersed or deflocculated colloids o-r colloidal slimes are then removed, by decantation, or otherwise,

from the ore pulp prior to the hydraulic classification or screening operation. As a result of this removal of the colloids in a dispersed o-r deflocculated state, the ore pulp is thereby freed from such colloidal matter, to the desired extent, so that the hydraulic classification or screening operation can thereafter be carried out without encountering the objections hereinbefore mentioned.

With the removal of the colloidal constituents there will, of course, also be removed a corresponding portion of the water of the ore pulp, together with such constituents as are dissolved therein. The. remaining ore pulp, which is accordingly in a more concentrated state, may be diluted, if desired, with colloid-free water to secure the proper .consistency for effective hydraulic classification and wet screening. The removal of the colloids from the ore pulp enables the ore pulp to be treated, if desired, in a state of greater density, that is, in a more concentrated form.

With lead-zinc sulfide ores I have used with advantage sodium silicate as the dispersing agent, but, in general, others agents, such as salts or acids, may be employed which will bringabout the desired dispersion ordefiocculation. The nature of the agent will vary somewhat with different ore pulps, because the colloidal slimes vary, in some cases being of a basic character and in others of an acid character, and in some cases being positively charged, and in other cases being negatively charged. The dispersing or deflocculating agent should be properly chosen with reference to the colloidal properties of the ore pulp and in ac cordance with the known principles of colloid chemistry, so that the desired deflocculation or dispersion will take place. Using sodium silicate, T have found that good results are obtained with an exceedingly small amount, for example, from four pounds down to a fraction of a pound of the sodium silicate per ton of dry ore treated. The dispersing agent or agents may thus be of an acid character, in case the colloidal constituents are such as are dispersed by an acid reagent, or of an alkaline character, if the preliminary tests indicate this to be desirable. The agent or agents may themselves &

I partake of a colloidal character, as, for example, silicic acid or sodium or potassium silicates, or soaps which also display acid or alkaline characteristics, or gum arabic (acacia) in small amounts may be added to promote the desired dispersion and separation of the colloids.

The dispersion of the colloids (whether adsorbed by the mineral particles or otherwise coagulated or flocculated) may be promoted, or even brought about, to a greater or less extent, by rapid circulation and agitation of the pulp, which will serve to distribute the deflocculating agent, if any is used, and cause abrasion of the mineral particles with each other and with fioccules or coagules of colloidal matter in which the mineral particles may beenmeshed, so that separated and dispersed colloids may be more readily removed by decantation. Variation in temperature may similarly be of assistance in bringing the colloids into a state permitting their removal from the ore ul p I have found that an extremely convenient and effective way of securing the necessary distribution of the dispensing or deflocculating agent through the mass of the ore pulp, and of securing the desired degree of dispersion or defiocculation of the colloids present in the pulp, consists in adding the deflocculating or dispersing agent to the pulp at some point prior to the admission of the ore pulp to the ball or pebble mills, or other fine grinding machines. Such machines almost invariably form a part of the equipment of ore dressing plants, as a preliminary step in the process of preparing ores for concentration or for the recovery of values.

By addingthe deflocculating or dispersing agent to the ore pulp ahead of the fine grinding mills, the necessary mixture is secured without inconvenience or added expense, and the dispersion or defiocculation of the colloids or colloidal slimes takes place under the most favorable conditions of violent agitation. Under t-lTQSG conditions fioccules or coagules are readily broken up and prevented from reforming and the particles already coated with colloids or enmeshed within fioccules are cleaned or released. Such colloids as are released from the solid or indurated condition by the grinding are immediately placed in the dispersed or deflocculated state, and the fresh mineral surfaces exposed by the grinding are prevented from being covered with coatings ofcolloidal material. This procedure may, with some ores, be suflicient to effect the desired degree of deflocculation, without the use of a deflocculating agent, provided the decantation of the colloidal constituents is efi'ected immediately after Whatever may be the preliminary treatment, or combination of treatments, to-

which the ore pulp is subjected for the dis-' persion or deflocculation of its colloids, these colloids are, according to the present invention, removed from the ore pulp, for example, by decantation, whe it is desired that the products of the subs-quent hydraulic classification or Wet screening shall be free of the colloidal constituents of the original ore pulp.

The colloidal slimes which are separated from the ore pulp, in accordance with the invention, are commonly of a character such that they can be run to waste. In case, however, these slimes carry sufficient values, they may be themselves subjected to a further treatment for the recovery of the values therefrom. The Water accompanying the removed colloids may also be separated and recovered for use. The colloidal constituents contained therein may thus be coagulated and settled or filtered out. The water thus separated from the colloids may be purified, if necessary, to free it from dissolved salts prejudicial to the maintenance of the desired state of colloidal dispersion in the ore pulp to which it may be subsequently added.

Where the ore pulp contains in solution in the water thereof constituents which prevent dispersion.- or deflocculation of the colloids, such constituents may be removed, as by the removal of the water from the ore pulp prior to the dispersion or deflocculation treatment, or they may be otherwise neutralized and rendered harmless by methods and agents which are generally known and can be regulated by simple tests. For example, an excessive quantity of magnesium sulfate or other electrolyte in the pulp interferes with the. proper action of sodium silicate as a dispersing agent. Such a condition may be corrected by washing the ore for the removal of soluble salts. Where the pulp is acid or wherethere is latent acidity due to the occlusion of sulfur dioxide, an alkaline agent may be added to correct the acidity, whereby the sodium silicate is enabled to act in the desired manner. The decantation may be carried out on the countercurrent principle by the use of a V and accompanying slimes remain to permit the handling of the pulp by pumping and its preparation for subsequent treatment.

While I have hereinbefore particularly described the removal of the dispersed colloidal constituents from the ore pulp by sedimentation and decantation, I do not Wish to limit myself to this particular method of removing the colloids. Other methods for accomplishing the desired result are available. For, example, I have found that colloidal constituents may be removed from an ore pulp by a flotation treatment, under conditions permitting the removal by flotation of the colloidal constituents to the extent desired, while substantially inhibiting the flotation of mineral particles. To this end the ore pulp containing the colloids is subjected to a flotation treatment with an appropriate frothing agent, the object of which is to produce a froth or scum containing the colloids, with a relatively small proportion of the valuable mineral or minerals contained in the: pulp, and to separate this colloid-bearing froth from the bulk of the ore pulp, after which the remaining ore pulp may be hydraulically classified or wet screened in any desired or customary manner. I have found rosin soap in solution in water a satisfactory frothing agent for this purpose, although other soaps and other frothing agents are available which.

are colloid-selective, rather than mineral-selective.

I claim 1. The improvement in wet processes of classifying comminuted material amociated with colloidal constituents in an ore pulp, which consists in removing colloidal constituents substantially alone from the ore pulp, said removal of colloidal constituents being preceded by the step of dispersing or deflocculating colloidal matter which would otherwise be present in the pulp in a flocculated condition, and thereafter subjecting the remaining ore pulp to a wet process of classification for effecting the desired segregation of said comminuted'material.

2. The improvement in wet processes of classifying comminuted material associated with colloidal constituents in an ore pulp, which consists in removing colloidal constituents substantially alone from the ore pulp by decantation in the course of which a portion of the water forming the pulp is also removed, said removal of colloidal constituents being preceded by the step of dispersing or deflocculating colloidal matter which would otherwise be present in the'pulp in a flocculated condition, and thereafter subjecting the remaining ore pulp to a wet process of classification for efiecting the desired segregation of said comminuted material.

3. The improvement in wet processes of classifying comminuted material associated with colloidal constituents in an ore pulp, which consists in removing colloidal constituents substantially alone from the ore pulp, said removal of colloidal constituents being preceded by the step of adding to the ore pulp a dispersing or deflocculating agent ing to the ore pulp a dispersing or deflocculating agent for promoting dispersion or deflocculation of colloidal constituents therein, and thereafter subjecting the remaining ore pulp to a. wet process of classification for effecting the desired segregation of said comminuted material.

5. The improvement in wet processes of classifying comminuted material associated with colloidal constituents in an ore pulp, which consists in removing from the ore pulp colloidal constituents substantially alone and in a dispersed or deflocculated condition, said removal of colloidal constituents being preceded by the step of dispersing or deflocculating colloidal matter which would otherwise be present in the pulp in a flocculated condition, and thereafter subjecting the remaining ore pulp to a wet process of classification for elfecting the desired segregation of said comminuted material. 7

6. The improvement in wet processes of classifying comminuted material associated with colloidal constituents in an ore pulp, which consists in removing from the ore pulp colloidal constituents substantially alone and in a dispersed or deflocculated condition, said removal of colloidal constituentsbeing preceded by the step of adding to the ore pulp a dispersing or defiocculating agent for promoting dispersion or deflocculation of colloidal constituents therein, and thereafter subjecting the remaining ore pulp to a wet process of classification for effecting the desired segregation of said comminuted material.

7. The improvement in wet processes of classifying comminuted material associated with colloidal constituents in an ore pulp, which consists in removing from the ore pulp colloidal constituents substantially alone and in a dispersed or deflocculated condition by decantation in the course of which-a portion of the water forming the pulp is also removed, said removal of colloidal con- 5 stituents being precededby the step of dispersing or deflocculating colloidal matter which would otherwise be present in the pul in a flocculated condition, and thereafter subjecting the remaining ore pulp to a Wet process of classification for efiecting the desired segregation of said comminutedmaterial.

- In testimony whereof I aflix m signature.

' WALTER O. BORC EBDT. 

